Jill, I think you & your mom will enjoy Alaska. There are so many more variables than just the questions you have, and to get the most from your trip, I suggest you visit a travel agent, preferably one who specializes in cruises and/or Alaska. (At the risk of advertising, I can offer my services, especially as I am in the Chicago area - if you are interested, let me know.) I can think of several cruise lines that would meet your needs, and I'm glad to hear you are planning on a balcony cabin - I always strongly suggest them for Alaska clients!
If you go during the cruise season (May through September) you are not likely to see the Northern Lights (otherwise known as the aurora borealis), which are spectacular light displays in space caused by solar particles interacting with the earth's magnetic field. They are usually more visible in the colder months, though we did see them over interior Alaska during a colder than normal September.
Temperatures can vary a lot. Toward the early or later parts of the cruise season, they will of course usually be cooler. In general, expect anything from 45 to 65 degrees for high temperatures, with a possibility of some 70's in midsummer. The climate there is moderated by the ocean currents, so the Alaskan coast doesn't get the extreme cold we can in the winter (especially at night). Also expect clouds & rain for at least part of your time there - look at sunshine as a bonus. Take layers, with a windbreaker and a raincoat/slicker as essential gear.